The present invention is directed to the field of pumps for pumping molten metal and, in particular, to using a pump for pumping molten metal into a shot sleeve for die casting metal parts. More specifically, the invention is directed to an apparatus for die casting high melting point alloys containing aluminum or magnesium and the like, and low melting point alloys such as those containing zinc.
Metal parts may be produced using xe2x80x9ccold chamberxe2x80x9d and xe2x80x9chot chamberxe2x80x9d die casting apparatuses. Cold chamber apparatuses employ a molten metal reservoir that is separated from the casting machine. Enough metal for one just casting is normally ladled by hand through a port of a small chamber referred to as a shot sleeve. Since this is done by hand it undesirably results in variation in the quantity of molten metal that is fed into the shot sleeve. A hydraulically actuated ram moves in the shot sleeve to force the molten metal under pressure into a die. As the ram advances, it seals the port and forces the charge into the die at pressures which may range from several psi to 60,000 psi or more. The molten metal cools in the chamber prior to injection into the die, thereby lending itself to description as a xe2x80x9ccold chamberxe2x80x9d process.
The hot chamber process is used for low melting point alloys such as zinc alloys and may employ, for example, a machine comprising a fixed cylinder having a spout firmly connected to a nozzle locked against a die cavity. A piston operating in the cylinder is raised to uncover an inlet port below the molten metal level in the pot. After the molten metal fills the interior of the cylinder, the piston is forced downward, which causes the molten metal to flow through the spout and into the die. Once the metal solidifies in the die the piston is withdrawn, the die is opened and the casting is removed. The die is then closed and the process repeated.
It is generally believed that better metallurgical castings result from use of the hot chamber process since the molten metal is not cooled as in the cold chamber process. However, numerous attempts have been made to develop a hot chamber apparatus for casting high melting point aluminum without widespread success. In view of difficulties presented in the hot chamber process, the industry could benefit from a cold chamber die casting apparatus which eliminates the risk of workers having to carry out the dangerous task of ladling molten metal, and from a process which produces an accurate charge of molten metal into the shot sleeve. An automated hot chamber die casting pump which is commercially usable is also desired.
In general, the present invention is directed to a pump for pumping molten metal. The pump includes a pump base comprised of non-metallic, heat resistant material. The pump base comprises an inlet opening, a wall forming a pumping chamber, a passageway that communicates the inlet opening with the pumping chamber, and an outlet opening that communicates with the pumping chamber. A piston made of non-metallic, heat resistant material is disposed in the pumping chamber. A connecting member is fastened to the piston. A valve permits and restricts flow of molten metal in the pump base. An actuator is connected to the connecting member and is adapted for effecting reciprocal movement of the piston in the pumping chamber. The piston has a coefficient of thermal expansion and configuration effective to enable it to expand into contact with the wall during its reciprocal movement in the pumping chamber.
One aspect of the invention relates to use of the inventive pump for die casting molten metal. The apparatus includes the aforementioned pump, a conduit extending from the outlet opening to a discharge location outside the base, and a device for injecting the molten metal into a die. The injection device includes a shot chamber that receives molten metal pumped from the conduit at the discharge location and a ram disposed in the shot chamber and adapted to direct molten metal in the shot chamber to a die for casting the molten metal. The conduit is either spaced from the shot chamber at the discharge location (e.g., cold chamber die casting) or the conduit is connected to the shot chamber at the discharge location (hot chamber die casting).
The invention resides in the use of a heat-expanded piston in the pump, which provides contact with the pump chamber wall resulting in more accurate charges and inhibition of molten metal from passing above the piston. This leads to improved safety and more effective performance. The pumping chamber wall and the piston comprise non-metallic, heat-resistant material such as ceramic material. Suitable ceramic material is selected from the group consisting of silicon carbide, silicon nitride and alumina. In one aspect of the invention, the connecting member comprises a material that expands more than the piston in the molten metal environment, thereby expanding the piston into contact with the pumping chamber wall.
Another aspect of the invention employs a piston comprised of a non-metallic, heat resistant material (e.g., ceramic) which has a coefficient of thermal expansion and configuration selected so as to expand the piston into contact with the pumping chamber wall. This device may employ a connecting member made of refractory material (e.g., the same material as the piston) in which case it does not expand more than the piston, or may utilize a connecting member which expands more than the piston as disclosed herein.
Referring to more specific features of the invention, a heat resistant gasket may be disposed in the pumping chamber. In one aspect of the invention the pumping chamber wall comprises an upper annular sleeve comprising non-metallic, heat resistant material disposed along a path of travel of the upper surface of the piston and a lower annular sleeve made of non-metallic, heat resistant material disposed below the upper cylindrical sleeve in the pumping chamber; a gasket comprised of heat-resistant material is disposed between the upper and lower sleeves. The piston moves along a predetermined axial region of the pumping chamber (i.e., stroke). In the first aspect of the invention, the metal connecting member engages the piston proximate to this region. That is, the contact between the connecting member and the piston is near an axial location in the pumping chamber where piston-sleeve contact is desired.
The present invention advantageously enables an accurate charge of molten metal to be delivered to the shot chamber, which improves the die casting process. In addition, the risk associated with ladling the molten metal by hand is avoided. The charge may be accurately varied as well, using stop member sleeves, a vertically movable stop plate and the like. The valve of the pump is advantageous in that it is a reliable and efficient way to regulate the charge into the pumping chamber.
The present invention is especially adapted for use in the cold chamber process, to replace the hand ladling that is often used. However, the present apparatus may be used in a hot chamber die casting process, by connecting the conduit with the shot sleeve or directly to a die without using a shot sleeve. In this case, a seal may be disposed around the piston that is suitable to enable sufficient pressure to be generated in the pumping chamber. In addition, the conduit may be heated or suitably insulated so as to prevent chilling of the molten metal prior to entering the die.
Many additional features, advantages, and a fuller understanding of the invention will be had from the accompanying drawings and the detailed description that follows.